1. Field of the Invention
The present invention relates generally to positioning devices, and more particularly to a device for rotatably positioning a camera about two orthogonal rotational axes.
2. Description of the Prior Art
Videoconferencing systems utilize video cameras to capture an image of the conference participants for transmission to a remote conferencing site. The video camera may be beneficially provided with a device for adjusting the rotational orientation of the camera such that the image of a speaking conference participant is properly framed. The rotational orientation of the camera may be adjusted manually by user manipulation of controls (located either on a videoconferencing system console or on a remote control device associated therewith) which cause the camera to rotate in the desired direction or directions. Alternatively and preferably, adjustment of the camera""s rotational orientation is achieved by an automatic technique such as acoustic localization, wherein the position of the speaking participant is calculated by analysis of plural acoustic signals, and the camera is steered in the direction of the calculated position.
Irrespective of whether the camera rotational positioning device is controlled by manual or automatic techniques, such devices require one or more actuators to rotate the camera such that the desired rotational orientation is achieved. Positioning devices designed to rotate the camera about two orthogonal axes (in order to enable more accurate framing of the image of speaking conference participants) typically utilize two actuators: the first actuator rotates the camera about a vertical axis, while the second actuator rotates the camera about a horizontal axis perpendicular to the camera""s longitudinal axis. Rotation of the camera about the vertical axis is referred to herein as xe2x80x9cpanningxe2x80x9d; rotation about the horizontal axis is referred to herein as xe2x80x9ctiltingxe2x80x9d; and, devices for rotating the camera about a vertical and horizontal axis are referred to herein as xe2x80x9cpan/tilt positioning devices.xe2x80x9d
Most presently-employed pan/tilt positioning devices utilize stepper motors to effect rotation of the camera. Stepper motors have the advantage of being relatively inexpensive, easy to control, and widely commercially available in a range of sizes and operational specifications. However, the use of stepper motors to rotate a camera in a videoconferencing system has a number of problems associated therewith. Typically, a gear train or similar set of transmission elements must be employed to transmit power from the stepper motor shaft to the camera. Movement of the gears during operation of the positioning device can generate a substantial amount of noise. This noise can be annoying and distracting to the conference participants. More importantly, the noise can interfere with acoustic localization techniques utilized to automatically orient the camera in the direction of the speaking participant.
Further, gears or other transmission elements may be susceptible to misalignment or breakage due to wear or rough handling, thereby rendering the positioning device partially or fully inoperative. Still further, the inclusion of the gear train into the positioning device makes its assembly more complex and consequently raises manufacturing costs. Finally, positioning devices utilizing stepper motors typically operate slowly and/or have limited angular resolution.
In view of the foregoing discussion, there is a need for a rotational positioning device which operates substantially noiselessly, does not require a gear train or similar to transmit power from the actuators to the camera.
The present invention comprises a rotational positioning device for a camera or similar article which utilizes voice coil actuators to drive rotation of the camera about first and second orthogonal rotational axes. The positioning device includes a carriage to which the camera is fixedly attached. The carriage is supported by a yoke such that it can pivot relative to the yoke about a horizontal axis orthogonal to the longitudinal axis of the camera (thereby allowing adjustment of the camera""s tilt angle). The yoke is in turn coupled to a base for pivotal movement relative to the base about a vertical axis (thereby allowing adjustment of the pan angle).
The positioning device advantageously employs first and second rotational actuators of the voice-coil type to respectively effect tilting and panning movements of the camera. As is known in the art, the voice coil actuators each include a set of permanent magnets and at least one coil wound on a non-ferrous base. Supplying current to the coil alters a magnetic field generated in a gap disposed between the magnets and the coil, thereby creating a force vector and causing the magnets to move relative to the coil (or the coil to move relative to the magnets, depending on whether the coil or the magnets are free to move). In a preferred embodiment of the invention, the first voice coil actuator is configured to tilt the camera, and comprises a vertically oriented coil, mounted to the yoke, interposed between a pair of return plates depending from the carriage. One of the return plates is provided with a pair of arcuate permanent magnets. When current is supplied to the coil, the carriage and supported camera are caused to rotate (tilt) relative to the yoke. The direction and speed of rotation is controlled by varying the direction and amount of current supplied to the coil.
In order to overcome rotational travel limitations associated with conventional rotational voice coil actuators, the second rotational actuator is preferably adapted with a dual-coil design. This design advantageously enables panning of the camera about an extended angular range (up to a full 360xc2x0). The second voice coil actuator preferably comprises a planar coil assembly supporting two opposed coils. The coil assembly is centrally mounted to a depending shaft of the yoke for co-rotation therewith, and is disposed in a gap between a base plate and a return plate of the base assembly. A pair of adjacently positioned arcuate permanent magnets are fastened to an underside of the return plate. Panning of the camera is effected by supplying current to the coils of the coil assembly, which causes the coil assembly and yoke to rotate relative to the base assembly. By appropriate commutation of the current supplied to the two coils, the camera may be panned over an extended angular range.
The positioning device may be further provided with a control system for precise control of the pan and tilt angle of the camera. The control system comprises a processor, a set of current supplies each configured to supply current to one coil of the first or second voice coil actuator, and angular position sensors for continuously detecting the pan and tilt angle of the camera. The processor is configured to calculate the desired pan and tilt angles of the camera based on acoustic localization data or the like and to determine the difference between the present pan and tilt angles (as measured by the angular position sensors). The, processor then directs each of the current supplies to supply current to the actuator coils so as to cause the positioning device to move the camera to the desired orientation.